As one of the major processes for forming patterns of LSI on a semiconductor substrate, the electron beam writing in which electron beam is irradiated to the wafer coated with photosensitive material to form circuit patterns is known. In the electron beam writer, an electrostatic chuck is used to hold and flatten the wafer in vacuum.
FIG. 1 is a cross-sectional view showing a typical electrostatic chucking electrode used in the conventional electron beam writer. The reference numeral 101 denotes a semiconductor wafer (hereinafter, referred to as wafer) to be a sample, 102 denotes a dielectric mainly made of alumina, and 103 denotes a chucking electrode buried in the dielectric 102. The chucking electrode 103 is connected to (+) side of a direct current power supply 105 via a switch 104. A holder 106 holds the wafer 101 by its surface so as to prevent the wafer 101 from floating. Meanwhile, a contact terminal (hereinafter, referred to as earth pin) 107 formed in the shape of a sharp needle is pressed to the rear surface of the wafer 101, and the wafer 101 is connected to the ground potential via the earth pin 107. A base pallet 108 is also connected to the ground potential.
More specifically, by applying direct voltage for the electrostatic chucking to the dielectric 102 between the wafer 101 and the chucking electrode 103 functioning as paired electrodes, the electric charge by the dielectric polarization is generated in the dielectric, thereby acquiring the electrostatic chucking force. This electrostatic chuck has a function to hold the sample so as not to move from a predetermined position and a function to flatten a wafer on which the convex and concave patterns of several tens μm are formed through processes such as the film formation and others so as to match with the flat chucking surface.
Note that, in FIG. 1, the earth pin is in contact with the rear surface of the wafer. However, the same function can be achieved even when it is in contact with the front surface thereof. Also, the chucking electrode is connected to (+) side of the power supply. However, the chucking function remains the same even when it is connected to (−) side thereof.
In this case, the wafer 101 is connected to the ground potential via the earth pin 107. Strictly speaking, however, since the current flowing in the wafer passes through the contact resistance formed at the contact portion between the wafer 101 and the earth pin 107, the potential difference occurs between the wafer 101 and the ground potential. The current flowing in the wafer indicates the leakage current from the chucking electrode 103 to the wafer 101 via the dielectric 102 and the beam current for the beam writing. For example, when the current flowing in the wafer 101 is 5 μA and the contact resistance between the earth pin 107 and the wafer 101 is 200 kΩ, the wafer 101 has the potential of 1 V.
As described above, when the wafer 101 is not kept at the ground potential, the trajectory of electrons as the charged particles is disturbed and the writing accuracy is degraded. J. Phys. E: Sci. Instrum. 14, 194 by M. Miyazaki (1981) discloses that the displacement of written patterns occurs because the amount of deflection of irradiated electrons changes depending on the potential of the wafer. Also, when the wafer 101 is not kept at the ground potential, the difference in potential occurs between the wafer 101 and the base pallet 108 which is kept at the ground potential. Therefore, the electric field which distorts the trajectory of electron beam irradiated to the wafer is generated near the wafer edge. As is evident from above, in order to obtain the high writing accuracy in the electron beam writer, it is necessary to keep the wafer 101 at the ground potential in the electrostatic chuck.
FIG. 4 of Japanese Patent Application Laid-Open Publication No. 2001-257158 discloses an electrostatic chuck in which the chucking electrode of the electrostatic chuck is divided into two parts, the direct current power supply is connected to each part, and an ammeter is connected in series between each direct current power supply and the ground potential. One of the direct current power supplies is a variable direct current power supply. The voltage is applied to the divided chucking electrodes and the voltage applied to the variable direct current power supply is adjusted so as to make the indication values of the ammeters equal to each other. By doing so, the two electrodes form a closed circuit. This closed circuit is formed in order to reduce the amount of leakage current generated between both chucking electrodes and the sample and flowing to the ground potential via the earth pin.
On the other hand, FIG. 1 and FIG. 3 of Japanese Patent Application Laid-Open Publication No. 11-111599 disclose the invention in which potential on the wafer surface is measured by using a surface electrometer to determine the correction voltage value to be applied to the earth pin or the chucking electrode. In this method, since the potential of the wafer is directly measured, not only the leakage current in the dielectric but also the wafer potential by the beam current can be measured at least in principle.
Furthermore, FIG. 2 of Japanese Patent Application Laid-Open Publication No. 11-111599 discloses the invention in which the current value flowing from the chucking electrode is measured and the correction voltage value to be applied to the earth pin is determined based on the measured value. In addition, FIG. 4 of Japanese Patent Application Laid-Open Publication No. 11-111599 discloses the technology in which two earth pins are provided to the wafer, and the difference in potential between one earth pin and the ground potential is measured, and then, the correction voltage value to be applied to the other earth pin is determined based on the measurement result. According to the invention described in Japanese Patent Application Laid-Open Publication No. 11-111599, the wafer potential can be reduced at least in principle.